Access Point/Bridge Protocol Data Units - Allen-Bradley Stratix 5100 User Manual

Chapter 9 Configuring Spanning Tree Protocol
Access Point/Bridge Protocol
Data Units
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STP defines a tree with a root bridge and a loop-free path from the root to all
infrastructure devices in the Layer 2 network.
STP discussions use the term root to describe two concepts: the bridge on the
TIP
network that serves as a central point in the spanning tree is called the root
bridge, and the port on each bridge that provides the most efficient path to the
root bridge is called the root port.
These meanings are separate from the Role in radio network setting that
includes root and non-root options. A bridge whose Role in radio network
setting is Root Bridge does not necessarily become the root bridge in the
spanning tree. In this chapter, the root bridge in the spanning tree is called the
spanning-tree root.
STP forces redundant data paths into a standby (blocked) state. If a network
segment in the spanning tree fails and a redundant path exists, the spanning-tree
algorithm recalculates the spanning-tree topology and activates the standby path.
When two interfaces on a bridge are part of a loop, the spanning-tree port
priority and path cost settings determine the interface that is put in the
forwarding state or the blocking state. The port priority value represents the
location of an interface in the network topology and how well it is located to pass
traffic. The path cost value represents media speed.
The access point supports both per-VLAN spanning tree (PVST) and a single
802.1q spanning tree without VLANs. The access point cannot run 802.1s MST
or 802.1d Common Spanning Tree, that maps multiple VLANs into a one-
instance spanning tree.
The access point maintains a separate spanning-tree instance for each active
VLAN configured on it. A bridge ID, consisting of the bridge priority and the
access point's MAC address is associated with each instance. For each VLAN, the
access point with the lowest access point ID becomes the spanning-tree root for
that VLAN.
The stable, active spanning-tree topology of your network is determined by these
elements:
• The unique access point ID (wireless access point priority and MAC
address) associated with each VLAN on each wireless access point
• The spanning-tree path cost to the spanning-tree root
• The port identifier (port priority and MAC address) associated with each
Layer 2 interface
Rockwell Automation Publication 1783-UM006A-EN-P - May 2014